Venezuela to Taiwan

Work inHawai’iandNew Zealand(and many other places) has shown the importance of soil age as a driver for ecosystem properties. But how can one quantify soil age if the age of the soil is not the same as the age of the rock upon which it sits? To give an example, the rocks underlying central Canada are over a billion years old, but the soils are not. Ten thousands years ago glaciers flowed over the landscape, removing all the soil, and thus any soil there today must be younger than 10,000 years. Erosion reset the clock. And erosion resets the clock even in places where glaciers have never been. Any hillslope will experience transport of soil down hill at some rate, and conversion of rock to soil at some rate. Together, these rates set a residence time for the soil (it’s even more complicated than that, because each element has it’s own residence time, but that’s another story).

Luckily, recent advances in geomorphology allow us to quantify how fast different catchments are eroding. It seems like one should just be able to tell by how steep it is, but it turns out places with similar steepness and climate can still be eroding at different rates. Instead, geomorphologists have turned to cosmogenic radionuclide analysis to quantify erosion rates, and with considerable success. The method works like this. Quartz grains near the Earth’s surface are bombarded with radiation from the atmosphere, and every once in a while, that radiation transforms a silica atom in that quartz into an exotic isotope of beryllium. The rate at which these transformations occur depend on three things: 1) latitude (more radiation towards the poles), 2) altitude (more radiation higher up) and 3) exposure time (e.g. how long a quartz grain sits near the surface before being eroded away). We know 1 and 2 for any given place, and 3 is what we want, it’s essentially the erosion rate.

With this in mind, my colleagueGeorge Hilleyand I set off to explore how erosion rates vary across tropical forests, and whether those erosion rates determine ecosystem properties such as soil fertility. We chose sites where the parent rock was the same (granite or granitic gneiss), and only looked at ridge tops so we didn’t have to worry about recent landslides or getting into depositional areas. We went from the Venezuelan Shield (very low erosion rates) to Taiwan (screaming fast erosion rates). We had sites with intermediate erosion rates (Costa Rica, Puerto Rico, Guatemala). We’re still working up the data, but we seem to have captured almost the entire range of erosion rates found on earth. Stay tuned for more as the papers come out…